In the control of systems which contain a non-linear element in a controlled primary signal path, it is sometimes necessary to inject one or more control tones to sense a feature of the non-linearity. For instance, a single tone can be used to describe the first derivative of the non-linearity and two tones through their intermodulation product can be used to describe the second derivative of the non-linearity.
The amplitude of the control tones used must be limited to a small percentage, typically less than 5%, of the amplitude of the primary signal passing through the control system or significant corruption of the primary signal will result. Further, the control tones must be spectrally isolated from the primary signal so that the control tones and their intermodulation products can be recovered for analysis or use in the control process.
An example of such a system may be found in U.S. Pat. No. 4,924,470, issued 8 May, 1990, and assigned to the same assignee as the present invention. This control system which was developed by one of the present co-inventors operates satisfactorily for the environment for which it was initially designed to operate.
In attempting to use the concept presented in the referenced application where some of the functions such as the NRZ-to-RZ function of block 10 are implemented on a gallium arsenide substrate, it was found that spurious oscillations originating within gallium arsenide integrated circuits would sometimes interfere with the operation of the control system. The spurious oscillations within a gallium arsenide integrated circuit vary in frequency depending upon various parameters such as temperature of the gallium arsenide circuit. Thus, it is substantially impossible to design filters into the circuit to eliminate the effect of the spurious oscillations.
When the presence of the interfering tone or spurious oscillation is close enough to the frequency of a control tone intermodulation product used in the control system to cause a beat note which is in the bandwidth of the control system, erratic behavior results in the system. To minimize the effect of the interfering tone or spurious oscillation, the present invention sweeps the frequency of the control tones so that the beat note developed between the intermodulation product and the spurious oscillation appears within the bandwidth of the control loop for only a small portion of the total sweep time. A preferred embodiment of the invention had a one Hz control loop bandwidth and the frequency of the control tone was swept over a range of ten Hz. Thus, the spurious oscillation interfered in the control system no more than 10% of the total time of operation. A well-designed signal detection portion of a control system will not be adversely affected when there are spurious interfering signals for only a small portion of time of its operation such as presented by the concept herein.
It is therefore an object of the present invention to provide an improved control system which is substantially imprevious to spurious interfering signals.
Other objects and advantages of the present invention will be apparent from a reading of the specification and the appended claims in conjunction with the drawings, wherein: